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Paired coral Sr/Ca and ¹⁸O records from the Chagos Archipelago: Late twentieth century warming affects rainfall variability in the tropical Indian Ocean

¹ Leibniz Institut für Meereswissenschaften, IFM-GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany
² International Pacific Research Center, University of Hawaii, POST Building 413, 1680 East West Road, Honolulu, Hawaii 96822, USA
³ Leibniz Institut für Meereswissenschaften, IFM-GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany
⁴ Institut für Geowissenschaften, Ludewig-Meyn-Strasse 10, 24118 Kiel, Germany

Understanding the relationship between sea surface temperature (SST) and precipitation is a significant challenge for climate models, particularly for the tropics. Here we present a new monthly coral Sr/Ca record from the tropical Indian Ocean (Chagos Archipelago) that extends from 1950 to 1995. The coral Sr/Ca ratio shows a stationary relationship with local SST, and documents a warming of 0.3 °C since 1950. Previous work has shown that the ¹⁸O values measured in the same coral core provide a proxy record of precipitation in the tropical Indian Ocean. The coral ¹⁸O record shows a nonstationary relationship with local SST, and a correlation between ¹⁸O and SST only emerges in the 1970s. It was proposed that this nonstationary behavior is due to an increase in mean SSTs in the tropical Indian Ocean. During the 1970s, SSTs reached a critical threshold (28.5 °C) beyond which small SST anomalies can have a significant impact on atmospheric convection. As a result, the covariance between SST and precipitation in the tropical Indian Ocean increased. Our new Sr/Ca data confirm that the warming of the Indian Ocean during the late twentieth century affects atmospheric convection and rainfall variability. Moreover, our proxy data show that the relationship between SST and precipitation is nonlinear and characterized by threshold behavior.

Key Words: Porites • paleoclimate • precipitation • trace elements • Indian Ocean • sea surface temperature • climate